Conventionally, a computer-based controller determines a current value of slip speed by calculating a difference between an engine speed and a turbine speed. The controller stores a retrievable desired slip speed table. This table contains desired values of slip speed mapped against various operating states of an automotive vehicle. These values are predetermined to meet torque multiplication demand and torque-variation suppression demand required for varying vehicle-operating state. The controller inputs information as to variables, such as engine speed, throttle position and vehicle speed, that describe current operating state of the vehicle and performs a table look-up operation of the desired slip speed table based on the input variable to determine a desired value of slip speed. It calculates a deviation between the current value of the slip speed and the desired value thereof. The controller alters a signal that provides a control authority over torque transmission by the lockup clutch in such a direction as to reduce the deviation toward zero.
Let us suppose that the vehicle travels with very low lead, that is, the vehicle travels with little engine output torque against low running resistance. In this operation state of the vehicle, the lockup clutch is released and thus there is no clutch engagement force with which the lockup clutch is engaged. However, the torque converter transmits torque with a slip speed in the neighborhood of zero. This phenomenon derives from the structure of the torque converter.
Let us now consider how the controller acts on the torque converter lockup clutch when the vehicle travels in the above-mentioned state.
As a current value of slip speed remains deviated from a desired value thereof, the controller keeps issuing a command that the clutch engagement force should drop. In response to this command, the lockup clutch engagement force drops to zero and the lockup clutch comes into its completely released position.
In order to save fuel when the vehicle is decelerating, the controller is programmed to operate the lockup clutch in complete engagement lockup mode. In the complete engagement lockup mode, the controller calculates the minimum clutch engagement force required to keep the lockup clutch engaged without inducing any slip. Considering a shift from the above-mentioned vehicle operation with very low load to the vehicle operation at deceleration, the lockup clutch has to move from the completely released position to a position whereat the complete engagement lockup mode begins. This travel of the lockup clutch accounts for a considerably long time taken until the initiation of the complete engagement lockup mode at deceleration. Fuel saving is not expected during this period of time. It would therefore be desirable to eliminate or at least shorten this transient period for increased fuel saving expected in the complete engagement lockup mode at deceleration.
Let us assume that the vehicle operator stamps on a gas pedal to accelerate the vehicle from the above-mentioned vehicle operation with very low load. In this case, the depression of gas pedal causes a temporal increase in a current value of slip speed. In order to restrain the current value of slip speed from increasing, the controller increases lockup clutch engagement force so as to reduce deviation between the current value of slip speed and a desired value thereof. In this process, the lockup clutch has to move from the completely released position to a position whereat the desired value of slip speed is accomplished. Thus, there occurs a period of time in which there is no gain in fuel saving expected by operation of lockup clutch in slip lockup mode for acceleration.
Closed loop or feedback control is known in which a slip error or deviation between a current value of slip speed and a desired value thereof is time integrated. According to this control, the absolute value of the result of time integration progressively increases infinitely when the vehicle travels with very low load.
With the feedback control including the integral term, as the absolute value of the integral term increases when the vehicle travels with very low load, the lockup clutch takes its completely released position. Thus, for a shift from the above-mentioned vehicle operation to vehicle operation at deceleration, the lockup clutch has to move from the completely released position to a position whereat the complete engagement lockup mode begins. Thus, the feedback control does not provide a solution to this problem.
Let us now assume that the vehicle operator stamps on a gas pedal to accelerate the vehicle from the vehicle operation with very low load. This causes an increase in current value of slip speed. Thus, the deviation of the current value of slip speed from a desired value thereof becomes negative. Even when the deviation becomes negative, if a current value resulting from integrating the deivation is sufficiently greater than the deviation, it takes a considerable time for the result of integration to sufficiently converge into a normal controllable region. During this transient period, the controller cannot perform any adjustment of current value of slip speed to a desired value thereof. Thus, it takes a considerably long time until occurrence of clutch engagement force with which the lockup clutch is engaged. As the period of time during which the current value of slip speed deviates from the desired value thereof is considerably long, the fuel saving is not improved as much as expected by operation of the torque converter in the slip control mode.
It is conceivable to map desired values of slip speed on a surface representing a change in running resistant to which the vehicle is subject and alters the desired values in accordance with variation of running resistance. Even if this method is employed, it is very difficult to eliminate uncontrollable region where current value of slip speed stays in the neighborhood of zero.
An object of the present invention is to improve a torque converter lockup control system such that the above-mentioned transient period is eliminated or at least minimized.